In the foundry industry, processes involving the use of shell molds and cores formed from sand and a resinous binder are well known. Normally, such processes involve intimately mixing sand particles with a small quantity of thermosetting resin, forming the mixture into a desired shape, and curing the resin in the mixture by heating the shaped mass at elevated temperatures. The resulting mold or core is of adequate hardness to shape molten metals poured thereover to form a metal cast.
Foundry cores and molds produced by the "hot-box process" in which the sand and binder are invested on a heated pattern, cured and ejected from the pattern are especially advantageous since this process can be carried out at high speed. For some time, the most important resin binders used in the "hot-box process" contained phenol-urea-formaldehyde resins or furfurly alcohol-urea-formaldehyde resins. In the presence of an acid catalyst, these resin systems cure rapidly and give cores and molds with high tensile strengths. Examples of the "hot-box process" using these binder compositions are disclosed in U.S. Pat. Nos. 3,059,297 and 3,480,573. Such binder systems are useful in many high-speed production operations where a high percentage of nitrogen from urea resins does not cause difficulties. In certain types of metal casting operations, however, high percentages of nitrogen can cause pinholing and other casting defects. Steel castings are particularly sensitive to nitrogen pinholing. For this reason, it is desirable to have foundry resin binders containing little or no nitrogen.
Resin binder systems with low nitrogen content have been disclosed in U.S. Pat. Nos. 3,725,333 and 3,755,229. These use a resin mixture which consists of furfurly alcohol and phenolic resole resin. In order to obtain cores or molds that cure quickly to high tensile strength, the acid salts, tin chloride or chromium nitrate, were used as catalysts with the resins. Although these systems give good cores and molds, they give rise to certain enviromental problems. The tin chlorides tend to give off irritating fumes during handling and their solutions are corrosive to metal and skin tissue. Chromium salts are highly toxic and chromium compounds are listed as priority toxic pollutants by the Environmental Protection Agency. Thus, for safety and ecological reasons, there is a need for a low-nitrogen resin binder system that can be cured with less-corrosive and less-toxic catalysts.
The nitrate salts of aluminum and iron are much more acceptable from the ecological viewpoint, but they are less effective catalysts for "hot-box" resin systems. It would, therefore, be an important advance in the art if a means could be provided to enhance the catalytic effect of these salts.